System and process for capture of acid gasses at elevated-pressure from gaseous process streams

ABSTRACT

A system, method, and material that enables the pressure-activated reversible chemical capture of acid gasses such as CO 2  from gas volumes such as streams, flows or any other volume. Once the acid gas is chemically captured, the resulting product typically a zwitterionic salt, can be subjected to a reduced pressure whereupon the resulting product will release the captures acid gas and the capture material will be regenerated. The invention includes this process as well as the materials and systems for carrying out and enabling this process.

STATEMENT REGARDING RIGHTS TO INVENTION MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with Government support under ContractDE-AC05-76RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to organic solvents that perform(pressure activated) chemically selective capture of acid gases fromgaseous product streams in the absence of water. More particularly, theinvention is a system and process for capture of CO₂ at elevatedpressure from gaseous process streams.

BACKGROUND OF THE INVENTION

Acid gases such as carbon dioxide have been implicated as major andrapidly expanding contributors to climate change over the last decade.As such, significant effort has been applied to the capture andsequestration of carbon dioxide (CO₂). CO₂ capture from pre-combustion,post-combustion, and flue gas sources, as well as contained human livingspace environments (e.g., submarines). Many of these existing systemsutilize aqueous solutions containing primary, secondary or tertiaryalkanolamines such as monoethanolamine (MEA) or methyl diethanolamine(MDEA) that chemically react with CO₂ and water to form thermally stablebicarbonate salts. However, aqueous solutions containing these captureagents have a low capture capacity (˜7 wt %) and thus readily reachsaturation. Additionally, these aqueous solutions are generallycorrosive to steel and other common materials of construction. Thiscorrosivity limits the alkanolamine concentration in water and requiresthe use of corrosion inhibitors. The limited alkanolamine concentrationrequires higher circulation rates and more energy expenditure for acidgas capture than would otherwise be necessary.

Physical absorbents are also commonly used as CO₂ capture agents, butare known to have a low selectivity for CO₂ unless CO₂ pressures arevery high and the gas stream has a large amount of CO₂. These physicalsorbents are often times irreversible or regenerable only aftersignificant thermal or chemical treatment. Non-amine based captureagents including, e.g., polyethylene glycol (e.g., Selexol®), cryogenicmethanol (e.g., Rectisol®), and N-methylpyrrolidone (e.g., Purisol®)also capture CO₂ via physical adsorption by dissolution into the liquid.However, these sorbents typically suffer from low weight capturecapacities (<10 wt %) and are typically used at total gas pressures near600 psig (41.2 atm). See Fundamentals of Natural Gas Processing, ArthurKidnay & William Parrish, CRC Press, Boca Raton, Fla. pages 100-104,110-113. Accordingly, new approaches are needed that solve CO₂selectivity and capacity issues associated with conventional captureagents and adsorbent technologies. The present invention meets theseneeds.

SUMMARY OF THE INVENTION

The present invention provides a system, method and materials thatenable the pressure-activated reversible chemical capture of acid gassessuch as CO₂ from gas volumes such as streams, flows or any other volume.Surprisingly, treating a dry gas stream using neat alkanolamines greatlyincreases the capture capacity of the amine and reduces the energyrequired for regeneration. In the case of CO₂, contact with theresulting product is typically but not always limited to a zwitterionicsalt, that can be subjected to a reduced pressure whereupon theresulting zwitterions decompose and release the captured CO₂ therebyregenerating the alkanol to its original active state. Surprisingly thezwitterionic salt like analogous ionic liquids has a disproportionatelyhigh solubility for CO₂ compared to aqueous solutions of alkanolamines,thus reducing the amount of these compounds need to capture a givenquantity of CO₂. This invention includes this process as well as thematerials and systems for carrying out and enabling this process.

In one embodiment the process involves contacting a gaseous volumecontaining CO₂ with at least one CO₂ binding organic compound containinga neat (water free) tertiary alkanolamine that chemically binds CO₂ toform a zwitterionic product at a pressure greater than ambient pressure,preferably greater than 100 psig; and removing the zwitterionic productfrom the gaseous stream or volume. If desired the zwitterionic productcan then be subjected to a reduction in pressure to release thechemically bound CO₂ and regenerate the CO₂ binding organic compound.

The equipment used for the gas liquid contact to absorb the acid gasesfrom the bulk gas stream is the same as that used conventionally for gasliquid contacting as is familiar to those skilled in the art. Examplesare gas/liquid counterflow absorption vessels containing an arrangementof trays, packing material, spray nozzles, and liquid distributors.Other examples are concurrent contactors such as Venturi scrubbers,spray towers; compact devices such as Higee contactors, or emulsifiers.Thus, this invention can incorporate any systems that can be used forefficient gas liquid contact. Similarly, for fluid regeneration andseparation of acid gas from the capture agent, pressure letdown valves,flash tanks, centrifugal devices, mist eliminators, and similarequipment used for separation of the acid gas from the liquid captureagent can be used.

Depending upon the exact desires of the user a variety of modificationsand alterations to this general embodiment may be had. In one embodimentthe binding organic compound is a liquid, selected from the groupconsisting of: N,N-Dimethylethanolamine (DMEA); N,N-Diethylethanolamine(DEEA), N,N-Diisopropylethanolamine (DIPEA);2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH); and combinationsthereof. In some applications the zwitterionic product is analkylcarbonate.

In another embodiment binding CO₂ in said gaseous volume or streamincludes using at least one CO₂ binding organic compound containing aprimary or secondary alcohol and a neat tertiary amine that when mixed,chemically binds CO₂ in said volume or stream to form a zwitterionicalkylcarbonate product that removes the CO₂ from the volume or stream.

A system for performing these methods includes at least one acid gasbinding organic compound (in the absence of water) that forms azwitterionic product when contacted by an acid gas at a pressure aboveambient that chemically binds and removes the acid gas from the volume.In one embodiment of the invention the acid gas binding organic liquidis an amine, preferably an alkanolamine. Examples of acid gas bindingorganic compounds include tertiary alkanolamines likeN,N-Dimethylethanolamine (DMEA); N,N-Diethylethanolamine (DEEA),N,N-Diisopropylethanolamine (DIPEA);2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH); or other types ofalkanolamines. These materials form zwitterionic products such as saltsand alkylcarbonates. In addition to these materials various alcohols andor polar/aprotic non-aqueous solvents such as but not limited todimethylsulfoxide, dimethylformamide, acetone, may also be included.

A preferred example of the material utilized as the reversible acid gascapture agent has the structure: Wherein n is any carbon-based chain andR and R₂ are any carbon-based chain or carbon containing alcohol. Thereversible acid gas capture agent reverts between a non-ionic form inthe absence of CO₂ to an ionic alkylcarbonate in the presence of CO₂under elevated pressures. The general reaction is shown below:

Various advantages and novel features of the present invention aredescribed herein and will become further readily apparent to thoseskilled in this art from the following detailed description. In thepreceding and following descriptions we have shown and described onlyone preferred embodiment of the invention, by way of illustration of thebest mode contemplated for carrying out the invention. As will berealized, the invention is capable of modification in various respectswithout departing from the invention. Accordingly, the drawings anddescription of the preferred embodiment set forth hereafter are to beregarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d show exemplary materials for use in system and process ofthe present invention.

FIG. 2 is a single component system showing chemical reaction between anexemplary tertiary alkanolamine and CO₂ at elevated CO₂ pressure.

FIG. 3 is a ¹³C NMR spectrum of a DMEA solution showing formation of azwitterionic DMEA-CO₂ alkylcarbonate species.

FIG. 4 shows the chemical and physical uptake of CO₂ in neat DMEAsolution as a function of pressure.

FIG. 5 plots conductance values in DMEA solution as a function of CO₂pressure.

FIG. 6 plots the conductance of DMEA as a function of CO₂ under repeatedcontact at pressures from 0 to 180 psi.

FIG. 7 shows a two component system involving reaction of an exemplarytertiary amine with a primary alcohol and CO₂ at elevated CO₂ pressure.

DETAILED DESCRIPTION

The following description includes the preferred best mode of oneembodiment of the present invention. It will be clear from thisdescription of the invention that the invention is not limited to theseillustrated embodiments but that the invention also includes a varietyof modifications and embodiments thereto. Therefore the presentdescription should be seen as illustrative and not limiting. While theinvention is susceptible of various modifications and alternativeconstructions, it should be understood, that there is no intention tolimit the invention to the specific form disclosed, but, on thecontrary, the invention is to cover all modifications, alternativeconstructions, and equivalents falling within the spirit and scope ofthe invention as defined in the claims.

In one embodiment of the present invention a material, system andprocess for pressure reversible selective chemical binding of CO₂ isdescribed. This invention allows the CO₂ to be chemically bound at apressure greater than ambient (STP) conditions and to be released bylowering the pressure. This pressure swing release enables the capturematerial to be regenerated to future use in a much more simplistic waythan in other applications that currently exist in the prior art. In oneexemplary embodiment neat alkanolamines are utilized to form a lowmolecular weight hybrid (chemical and physical) CO₂ scrubber thatchemically captures CO₂ and regenerates the capture agent using apressure-swing, providing an attractive gas capture system from thevantage point of chemical selectivity, weight capacity, and non-thermalregeneration. The chemical selectivity provided by the invention forcapture of CO₂ is coupled with the ease and energy savings provided bypressure reversal for release and recovery of CO₂.

The invention could be utilized in a variety of applications includingnatural gas sweetening (decontamination) and other CO₂ scrubbingprocesses. Because CO₂ scrubbing processes from natural gas operateunder elevated pressures, e.g., from about 300 psi to about 1,000 psi.the ability to absorb CO₂ at these elevated pressures combined withfacile release at standard temperature and pressure (STP) constitutes amodel technique for capture and recovery of CO₂ from such sources.Further, the ability to release CO₂ under pressure saves money forcompression costs for sequestration. The invention provides the firstpressure-reversible zwitterionic liquid that can provide directreplacement for conventional CO₂ capture processes. Two embodiments ofthe invention are described hereafter.

In one embodiment, organic CO₂ binding liquids containing neat tertiaryalkanolamines include both amine and alcohol functionalities in a singlestructural moiety (i.e., single component systems). Single organic CO₂binding liquid systems are preferred over dual component systemsdescribed hereafter containing an amine and an alcohol as separatecompounds due to their lower vapor pressures, which are better suited toindustrial applications. However in other applications otherconfigurations may be desired and appropriately created.

FIG. 1( a)-1(d) shows exemplary structure of tertiary alkanolamines thatcapture CO₂ at elevated pressures. Exemplary tertiary alkanolaminesinclude, but are not limited to, e.g., N,N-Dimethylethanolamine (DMEA),N,N-Diethylethanolamine (DEEA), N,N-Diisopropylethanolamine (DIPEA) and2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH). These liquidcompounds are available commercially. Neat N,N-Dimethylethanolamine(DMEA) shows marked CO₂ capture capacity and is expected to be anefficient CO₂ capture agent for industrial applications.

FIG. 2 shows the reaction scheme of a single component system involvingthe chemical reaction between an exemplary tertiary alkanolamine (DMEA)with CO₂ at elevated CO₂ pressure in the absence of water. Ethanolaminestested in conjunction with the invention were purified via distillationand dried/stored over 3 Å molecular sieves to remove water. Toinvestigate the STP binding efficiency of ethanolamines, neat solutionsof each were bubbled with CO₂ for 1 hour. ¹³C-NMR and conductivityexperiments allow for quantitative and qualitative measure of DMEAabsorption of CO₂, both chemical and physical, as well as regenerationof the DMEA from the bound form (DMEA-CO₂) upon simple depressurization.

FIG. 3 shows a typical ¹³C-NMR spectrum of a DMEA solution showingformation of a zwitterionic DMEA-CO₂ alkylcarbonate species, evidencedby peaks positioned at 125 ppm and between 156 and 158 ppm,respectively, which are attributed to: 1) dissolved CO₂ and 2) azwitterionic alkylcarbonate DMEA-CO₂ moiety, respectively. The ¹³C NMRspectrum of this solution shows peaks at 125 ppm and 156 ppm, which areattributed to dissolved CO₂ and the zwitterionic alkylcarbonate DMEA-CO₂(shown in FIG. 2), respectively. Under STP conditions, none of thesematerials in the absence of water absorbed CO₂, physically or chemicallyat standard temperature and pressure, as determined by gravimetricuptake and/or ¹H/¹³C NMR spectroscopy. At elevated pressures (100-500psi), however, DMEA successfully captures CO₂ via two modessimultaneously: chemical binding as the zwitterion, DMEA-CO₂, andphysical absorption.

FIG. 4 shows the chemical and physical wt % of CO₂ uptake in neat DMEAsolution as a function of pressure. TABLE 1 lists calculated values forchemical carboxylation and physical absorption as a function ofpressure.

TABLE 1 Carbon dioxide uptake in neat DMEA at various pressures. CO₂Chemical Physical pressure absorption^(a) absorption^(a,b) TotalMaterial (psi)/ (MPa) X_(CO2) ^(c) wt. % X_(CO2) ^(c) wt. % mole % wt. %DMEA 100/0.69 0.169 7.7% 0.060 2.9% 22.9% 10.6% 200/1.38 0.218 9.7%0.145 6.7% 36.3% 16.4% 300/2.07 0.244 10.7% 0.204 9.1% 44.8% 19.9%500/3.45 0.260 11.4% 0.191 8.6% 45.1% 20.0% ^(a)calculated from ¹³C NMRintegrations of pressurized reactions; values are the average to twoexperiments. ^(b)moles or grams of physically absorbed CO₂ divided bythe sum of absorbed CO₂, DMEA and DMEA-CO₂. ^(c)mole fraction.

Chemical carboxylation was calculated by integration of the relative—CH₂O— carbons of DMEA-CO₂ and DMEA, respectively. In the figure,formation of the DMEA-CO₂ moiety increases as a function of applied gaspressure. Results in TABLE 1 show that DMEA chemically captures up to7.7 wt. % carbon dioxide at pressures as low as 100 psi and 9.7, 10.7and 11.4 wt. % at 200, 300 and 500 psi respectively. Physically absorbedCO₂ also increases with increased gas pressure, exhibiting 2.9 wt. % at100 psi to 6.7, 9.1 and 8.6 wt. % at 200, 300 and 500 psi, respectively.Because carbon dioxide shows relatively high solubility in ionic liquidsand zwitterionic liquids, increasing the ionic nature of theDMEA/DMEA-CO₂ moieties in solution at higher pressures may facilitatephysical CO₂ absorption.

As shown in TABLE 1 and in FIG. 4, at 100 psi the amount of physicallyabsorbed CO₂ is approximately one-third that of the chemically absorbedCO₂. At elevated pressures, the same ratio (physical absorbed?) istwo-thirds or higher. The combined chemical/physical CO₂ capacity ofDMEA is 10.6 wt. % at 100 psi followed by a significant jump to 16.4,19.9 and 20 wt. % at 200, 300 and 500 psi, respectively. For reference,CO₂ capacities were compared with CO₂ capacities from conventionalcapture agents including, e.g., dimethyl (poly)ethylene glycol DEPEG,because of the similarity of DEPEG to SELEXOL®. TABLE 2 compares capturecapacities for uptake of CO₂.

TABLE 2 Comparison of the CO₂ uptake capacities of DMEA and DEPEG, aSelexol ™ derivative. CO₂ Total CO₂ Absorption pressure DMEA DEPEG^(a)(psi) X_(CO2) ^(b) wt. % X_(CO2) ^(b) wt. %^(c) 100 0.229 10.6% 0.18 3%200 0.363 16.4% 0.29 5% 300 0.448 19.9% 0.37 7% 500 0.451 20.0% 0.5513%  ^(a)Data taken/calculated from Gainar et al. (Fluid PhaseEquilibr., 1995, 109, 281). ^(b)mole fraction. ^(c)Estimated fromaverage molecular weight of mixture.

As shown in the table, at lower pressures (≦300 psi), DMEA absorbsappreciably more CO₂ than DEPEG per mole of solvent while at 500 psiDMEA shows evidence of an absorbance plateau. DMEA exhibits asubstantial capacity advantage for CO₂ over DEPEG (1.5× to 3.5×) atlower pressures. Thus mole capacities of DMEA rival those of DEPEG, aSelexol® derivative, at pressures ≦300 psi, while the weight capacitiesof DMEA is higher than those of DEPEG up to 500 psig. While results showDMEA is limited to ˜20 wt. % CO₂ uptake, this feature adds to theutility of the material. As the zwitterionic salt remains dissolved inthe DMEA solution (˜3:1 ratio of DMEA:DMEA-CO₂ at a chemical molefraction of 0.26), overall solution viscosity remains relatively lowsuch that the mixture can be pumped through capillary tubes withdiameters as small as 300 μm. Further, when DMEA/DMEA-CO₂ solutions aredepressurized, rapid decarboxylation occurs and the mixture cleanly andeasily reverts to DMEA. This is evident by the disappearance of thealkylcarbonate and dissolved CO₂ peaks and persistence of the DMEAsignals in ¹³C NMR spectroscopy. Thus, DMEA represents a CO₂ sorbentwhich effectively absorbs CO₂ both chemically and physically underpressure and successfully decarboxylates at STP to yield DMEA, avoidingthe need for costly thermal regeneration.

Of the chemical and physical absorption of CO₂ into DMEA, only chemicalreaction leads to a solution whose conductance is significantly altered.Effect of chemical CO₂ addition on solvent polarity was measured by theconductivity of DMEA over a CO₂ atmosphere at various pressures.Anhydrous DMEA showed a conductance of 3 μS/cm when introduced to ahigh-pressure conductance cell. The cell was pressurized with CO₂ at 15psi increments and the solution conductance was recorded.

FIG. 5 plots conductance values in a DMEA solution as a function of CO₂pressure. Diffusion and/or chemical addition of CO₂ into DMEA provedslow at low pressures; at 15 psi, vigorous stirring for 18 hours wasrequired to reach chemical equilibrium (i.e., established by anunvarying conductance). However, equilibrium was attained more rapidlyat higher pressures, an observation attributed to physical CO₂saturation reached at lower pressures. At a pressure of from 45 psi to60 psi, about 3.5 hours was required to reach equilibrium; less than 30minutes was needed to reach equilibrium at pressures of 150 psi, 165psi, and 180 psi. Conductance of the solution rose from 3 μS/cm to 890μS/cm at CO₂ pressures from 0 psi to 180 psi. At each pressure increase,temperature of the cell briefly increased by 3° C. to 6° C., indicatingthe chemical fixation of CO₂ by DMEA is slightly exothermic or the heatof dissolution is exothermic. The significant increase in conductance asa function of pressure confirms that the interaction of DMEA with CO₂involves a chemical reaction to form an ionic species and not simply aphysical dissolution.

The ability to regenerate DMEA was also tested. Anhydrous DMEA wascarboxylated with CO₂ at a pressure of 180 psi and depressurized overfive cycles. FIG. 6 plots the conductance of DMEA for the repeatedcontacts with CO₂ at pressures ranging from 0 to 180 psi. As shown inthe figure, conductance of the DMEA:DMEA-CO₂ solution for each cyclerepeatedly reaches 890 μS/cm (±4 μS/cm) at 180 psi and falls to 42 μS/cm(±3 μS/cm) at 0 psi. A small residual conductance (42 μS/cm) observed atthe end of each cycle was reduced to 7 μS/cm following an N₂ purge ofthe solution, confirming that complete chemical decarboxylation can easybe achieved. These conductance measurements yield no discernabledeterioration in the chemical CO₂-binding uptake capacity of DMEA fromrepeated carboxylation/decarboxylation cycles. While this experimentdoes not unambiguously verify repeatable physical carbon dioxide uptakeby DMEA, we surmise that gaseous dissolution remains unchanged afternumerous cycles based on the chemical uptake repeatability. TABLE 3compares carboxylation properties of DMEA, DEEA, DIPEA, and 2-DMAM-PrOH,respectively.

TABLE 3 Carbon dioxide uptake by anhydrous alkanolamines at elevatedpressures (25° C.). CO₂ Chemical^(a) Physical^(a,c) Total Select ¹³CNMRMaterial (psi). X_(CO2) ^(b) wt. % X_(CO2) ^(b) Wt. % wt. % signals(ppm)^(d) DMEA 300 0.244 10.7% 0.204 9.1% 19.8% 62.3 (—CH₂OCO₂—), 156.23(—OC(O)O—) DEEA 300 0.209  7.2% 0.223 8.7% 15.9% 62.7 (—CH₂OCO₂—), 158.5(—OC(O)O—) DIPEA 300 0.140   4% 0.430  12%  16% 66.1 (—CH₂OCO₂—)_(,)159.1 (—OC(O)O—) 2-DMAM-PrOH 300 —^(e) —^(e) —^(e) —^(e) —^(e) 70.0(—CH₂OCO₂—), 158.5 (—OC(O)O—) ^(a)calculated from ¹³C-NMR integrationsof pressurized reactions; values are the average to two experiments.^(b)mole fraction. ^(c)moles/grams of physically absorbed CO₂ divided bythe sum of absorbed CO₂, chemically bound alkanolamine and freealkanolamine. ^(d)referenced to dissolved CO₂ set at 125 ppm except for2-MDMA-PrOH. ^(e)could not be determined; see text for details.

As shown in TABLE 3, DMEA shows significant CO₂ uptake at pressures from100 psi to 500 psi. Other alkanolamines listed in the table (e.g., DEEA,DIPEA and 2-DMAM-PrOH) exhibit distinctly different carboxylationproperties under similar conditions.

Increasing the functionalization of the amine moiety withelectron-donating substituents that increase basicity was found todecrease the chemical binding capacity for CO₂. For example, resultsshow chemical binding of CO₂ decreases from DMEA to DEEA to DIPEA byfrom about 4 to 5 mole % each. While chemical binding of CO₂ is observedfor 2-DMAM-PrOH, CO₂ pressurization of this low melting point (mp=19-20°C.) liquid results in partial solidification that precludes accuratemeasurement of CO₂ uptake capacities via NMR spectroscopy. To accountfor the decreasing chemical binding trend for CO₂ of DMEA>DEEA>DIPEA,relative polarity effects of these solvents were considered. Asdescribed herein, chemical binding of CO₂ by DMEA, DEEA, or DIPEAresults in the formation of highly polar zwitterions, whereas theorganic solvents themselves have relatively low polarities.Stabilization of the polar, highly charged zwitterions is thus impactedby the intrinsic polarity of the solvent medium. TABLE 4 listsabsorption maxima for Reichardt's dye used as a molecular probe (givenits acute absorption maximum sensitivity to small polarity changes) toassess polarity in the DMEA, DEEA, and DIPEA media measured using UV-VISspectroscopy, along with absorption maxima in other common organicsolvents.

TABLE 4 Absorption maxima of Reichardt's dye in alkanolamines DMEA, DEEAand DIPEA and select common organic solvents. Solvent TOL CH2Cl2 DIPEACH3CN DEEA i-PrOH DMEA EtOH MeOH Reichardt's 806 692 624 617 605 592 580550 516 dye, λ_(max) (nm) —————————— increasing polarity ——————————→

DMEA (with the smallest N-substituents) is the most polar of theselected alkanolamines, followed by DEEA, and then DIPEA, whose bulkieraliphatic N-substituents decrease the polarity of the solvent. Theincreasing chemical binding capacity for CO₂ of DMEA>DEEA>DIPEA isattributed to more effective stabilization of the correspondingzwitterionic alkylcarbonate associated with the increasing solventpolarity. This stabilizing polarity effect overshadows the basicityeffect of the alkanolamines, highlighting an important principle forthese types of liquids with regard to chemical CO₂ binding capacity. Forphysical uptake of CO₂, the opposite trend is observed, withDIPEA>DEEA>DMEA. CO₂ shows greater physical solubility in aliphatic,non-polar organic solvents than in polar media. Here the increasedphysical absorption of CO₂ in DIPEA over the more polar DMEA and DEEA isattributed to the affinity of dissolved CO₂ for non-polar organicsolvents.

In another embodiment of the invention, a two component system for CO₂capture involves a tertiary amine (e.g., triethylamine) paired with aprimary or a secondary alcohol at elevated pressures (above STP) to formammonium alkylcarbonate ionic liquids, as shown in Equations [1] and[2]:

CO₂+Base+ROH

BaseH⁺ROCO₂ ⁻  [1]

K_(eq)=[BaseH⁺][ROCO₂ ⁻]/P_(CO2)[Base][ROH][  2]

Tertiary amines show little-to-no binding of CO₂ in combination withalcohols at STP. Thus, captured CO₂ can be easily stripped bydepressurizing the system.

FIG. 7 shows the carboxylation of methanol with tertiary amines andother bases at elevated CO₂ pressures, e.g., near 10 atm. TABLE 5 showsthe reactivity of methanol with several exemplary tertiary amines andother bases at the elevated CO₂ pressure of 10 atm.

TABLE 5 Carbonation of methanol with various tertiary amines IR (C═O)¹³C NMR pKa R₃ Conversion^(a) cm⁻¹ ppm (DMSO) Triethylamine 75% 165452.3^(b) 9.0 160.1^(c) Diisopropylethylamine 92% 1647 51.1 18.6 158.8(MeCN) DABCO^(d) 64% 1650 49.1 8.9 160.0 DMAP^(e) 64% 1651 52.4 160.5DBU^(f) 98% 1642 52.2 24 159.8 (MeCN) 2,6-Lutidine No reaction ~4Pyridine No reaction 3.4 ^(a)Conversion based on amine as determined byin situ ¹³C NMR of 2M base in methanol under 10 atm CO₂. ^(b)Resonanceof methyl carbon in CH₃OC(O)O⁻. ^(c)Resonance of carbonate carbon inCH₃OC(O)O⁻. ^(d)1,4-diazabicyclo[2.2.2]octane.^(e)4-(dimethylamino)pyridine. ^(f)1,8-Diazabicyclo[5.4.0]undec-7-ene.

Tertiary amines produced ammonium methylcarbonate salts at a highconversion (˜159 ppm ¹³C NMR). The alkylcarbonate peak is indicative ofthe chemical binding of CO₂ (as compares with physical dissolution,which involves a CO₂ peak at 125 ppm). At the pressures used in thisstudy, there was also substantial dissolved CO₂ observed in the ¹³C NMRspectra. The strongest bases such as DBU and Diisopropylethylamine(Hünig's base) showed the highest conversion, followed by TEA, DABCO andDMAP. Hünig's base, which has the same basicity as TEA, has 17% morebound CO₂. DABCO and DMAP bind less CO₂ likely due to steric bulk.Lutidine and Pyridine showed no reactivity to form alkyl carbonates atthis pressure most likely due to their much-reduced basicity compared totertiary amines. Ammonium alkylcarbonate salts listed in TABLE 5decompose back to the corresponding amine, methanol, and CO₂ upon returnof CO₂ pressure to atmospheric conditions, however stronger bases suchas DBU need thermal regeneration and do not decarboxylate upon reductionin pressure.

Carboxylations were also performed in MeCN solvent rather than methanol.TABLE 6 lists carbonation results of various alcohols with TEA and CO₂in MeCN.

TABLE 6 Carboxylation of various alcohols with triethylamine and CO₂ RConversion^(a) CH₃— 75% CH₃CH₂— 75% 1-octanol 25%

65%^(b)

35%^(c) i-Propanol 25% t-Butanol No Reaction Phenyl No Reaction a)Conversion based on NEt₃. b) Initial product is ClCH₂CH₂OCO²⁻. Thisproduct cleanly cyclizes to ethylene carbonate. c) Reaction wasperformed in CH₃CN. Only one hydroxyl is carbonated.

Primary and secondary alcohols readily convert to corresponding alkylcarbonates, whereas tertiary alcohols do not. Results are attributed tosteric crowding of the alcohol. Data also show the degree ofcarboxylation of the alcohol decreases as the alcohol chain lengthincreases and subsequently becomes less polar. The decrease in polarityis attributed to the lack of a polar solvent that can stabilize thetransition states of the molecules during the carboxylation process. Forexample, trifluoroethanol (considered to have a steric bulk equivalentwith that of ethanol, but with a much lower pKa) is unreactive towardCO₂. Phenol doesn't carboxylate under these conditions, which is likelydue to it being too acidic to bind CO₂. Data in TABLES 5 and 6 suggestshort linear alcohols and tertiary amines are preferred combinations fora high-pressure CO₂ capture solvent system, but is not limited thereto.

A first challenge in designing neat trialkylamine and alcohol blends toperform capture in the absence of solvent is to use alcohols and basesthat are non-volatile and to form liquid ammonium alkylcarbonates (notsolids as in the case of methanol) that are cheap. We set out to find anon-volatile tertiary amine and alcohol that would mitigate materialloss and improve costs. The amine and alcohol were bifunctionalized tomake them less volatile. As CO₂ is introduced over an organic molecule,it causes volumetric expansion and a decrease in polarity as the molefraction of CO₂ increases. CO₂ binding requires a highly polar medium tostabilize the polar transition states and the zwitterionicalkylcarbonate. Polarity data measured for alkanolamines demonstratesthat a CO₂ pressure near 150 psi decreases the polarity. A drop inpolarity promotes dissolution of CO₂ into the alkanolamine, not thedesired chemical binding. DMEA however is sufficiently polar tostabilize these polar species and subsequently is a good candidate toreact with CO₂ at a low pressure (i.e., 150 psi) condition.

Decomposition of the alkylcarbonate salts by depressurization is highlyadvantageous for high-pressure CO₂ gas capture as the pressure swingavoids use of an energy intensive thermal solvent regeneration cycle.The demonstrated pressure desorption of the chemically bound CO₂ fromammonium alkylcarbonates parallels the energy requirements for therelease of physically absorbed CO₂ by industrial materials such asSELEXOL® and RECTISOL®. These high-pressure anhydrous alkanolamines canpotentially be superior to physical sorbents because they contain theeconomical pressure swing yet they contain a highly chemically selectiveCO₂ capture.

The following example provides a further understanding of the invention.Dissolved CO₂ appears at 125 ppm in ¹³C NMR spectra in multiple organicsolvents while the alkylcarbonate (R—O—CO₂ ⁻) peak appears at 158 ppm.The carbonyl signal is attributed to the alkylcarbonate, as carbamatesdo not form for the tertiary amine. While primary and secondary aminescan and do react with carbon dioxide to yield carbamates, tertiaryamines do not react directly with CO₂, although bicarbonate salts canform in the presence of water. ¹³C NMR chemical shifts confirm thatchemical CO₂ binding to an alkanolamine (in the absence of water)proceeds via the alcohol moiety of DMEA as opposed to the tertiaryamine. The appearance of a resolved —CH₂O— methylene signal at 63 ppm,downfield from the alcohol methylene of DMEA, is indicative of theeffects of β-carboxylation as opposed to N-carboxylation. To investigatethe extent to which DMEA will absorb CO₂, neat solutions of DMEA werepressurized at 100, 200, 300 and 500 psi of CO₂ for 18 hours, loadedinto a PEEK NMR tube and analyzed. Carboxylation experiments at higherpressures were not performed as CO₂ begins to liquefy above 500 psi.Under these conditions CO₂ is likely to phase separate from theDMEA/DMEA-CO₂ mixture and hamper NMR interpretation. Both DMEA-CO₂ anddissolved CO₂ were observed at all pressures and the relative quantitiesof each calculated from ¹³C NMR integrations. Extent of both chemicalcarboxylation and physical absorption as a function of pressure werecalculated (see TABLE 1 and FIG. 4) using the relative integration ofthe —CH₂O— carbons of DMEA-CO₂ and DMEA. Physical absorption was foundby comparing the relative integration of the carbonyl carbons of CO₂ andDMEA-CO₂.

Neat solutions of alkyl ethanolamines (DMEA, DEEA, and DIPEA) absorb andchemically bind carbon dioxide at elevated pressures by formation ofalkylcarbonates. Through both chemical binding and physical absorptionDMEA captures up to 45 mole % (20 wt. %) carbon dioxide, while DEEAcaptures up to 43 mole % (16 wt. %) and DIPEA captures up to 57 mole %(16 wt. %) carbon dioxide (300 psi). The increasing chemical uptakecapacity trend of DMEA>DEEA>DIPEA is attributed to solvent polarityeffects while the physical CO₂ absorption trend of DIPEA>DEEA>DMEA isexplained by the affinity of carbon dioxide for non-polar organic media.DMEA shows the greatest wt. % uptake of carbon dioxide and chemicallybinds CO₂ under pressure more effectively than the other tertiaryethanolamines to form the thermodynamically unstable zwitterionicalkylcarbonate salt DMEA-CO₂. DMEA captures up to 45 mole % (20 wt. %)of CO₂ at 500 psi via combined chemical binding and physical absorption.Carbon dioxide weight capacities of DMEA rival those of DEPEG, aSELEXOL® derivative, at pressures 300 psi. DMEA-CO₂, DEEA-CO₂ andDIPEA-CO₂ are characterized by high-pressure ¹³C NMR and give rise to¹³C resonances analogous to previously studied zwitterionicalkylcarbonates. The zwitterion DMEA-CO₂ regenerates CO₂ and DMEA upondepressurization. This natural decarboxylation when pressure is releaseis advantageous for high-pressure gas-capture systems as the sorbent canbe regenerated by an economical pressure swing as opposed to a morecostly thermal swing. Repeated CO₂ absorption/release experiments showno decline in the chemical binding CO₂ uptake capacity of DMEA over 5cycles.

Tertiary amines combined with alcohols chemically and selectively bindCO₂ under mild pressures to form thermodynamically unstablealkylcarbonate salts. The carboxylations of numerous amine and alcoholpairs can be tracked in situ using IR and NMR spectroscopy.Alkanolamines also capture CO₂ under elevated pressures as zwitterionicalkylcarbonate salts. The degree of alcohol carboxylation is limited bythe polarity of the solvent as well as the basicity of the amine.Ammonium alkylcarbonate salts decarboxylate into their correspondingalcohols and amines unless under pressures near 10 ATM. This naturaldecarboxylation when pressure is released is advantageous forhigh-pressure gas-capture systems because the sorbent can be regeneratedby an economical pressure swing instead of the more costly thermalswing.

While preferred embodiments of the present invention have been shown anddescribed, it will be apparent to those of ordinary skill in the artthat many changes and modifications may be made with various materialcombinations without departing from the invention in its true scope andbroader aspects. The appended claims are therefore intended to cover allsuch changes and modifications as fall within the spirit and scope ofthe invention.

What is claimed is:
 1. A method for removing CO₂ from a gaseous volumecomprising the steps of: contacting a dry gaseous volume containing CO₂with at least one CO₂ binding organic compound containing a neat(anhydrous) tertiary alkanolamine that chemically binds CO₂ to form azwitterionic product at a pressure greater than 100 psi and removingsaid zwitterionic alkylcarbonate product from said gaseous stream orvolume.
 2. The method of claim 1 further comprising the step ofdepressurizing said zwitterionic alkylcarbonate to release saidchemically bound CO₂ and regenerate said CO₂ binding organic compound.3. The method of claim 1, wherein said binding organic compound is aliquid.
 4. The method of claim 1, wherein said tertiary alkanolamine isselected from the group consisting of: N,N-Dimethylethanolamine (DMEA);N,N-Diethylethanolamine (DEEA), N,N-Diisopropylethanolamine (DIPEA);2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH); and combinationsthereof.
 5. The method of claim 1, wherein the zwitterionic product isan alkylcarbonate.
 6. A method for chemically binding and removing CO₂from a gaseous volume comprising the step of: binding CO₂ in saidgaseous volume using at least one CO₂ binding organic compoundcontaining a neat tertiary amine that when mixed with a primary orsecondary alcohol chemically binds CO₂ in said volume to form azwitterionic alkylcarbonate product that removes the CO₂ from saidvolume or stream.
 7. A system for removal of CO₂ from a gaseous volumeor stream, characterized by: at least one CO₂ binding organic compoundcomprising a neat tertiary alkanolamine that forms a zwitterionicproduct when contacted by CO₂ at a pressure above ambient thatchemically binds and removes CO₂ from said volume.
 8. The system ofclaim 7, wherein the total system pressure is 100 psi.
 9. The system ofclaim 7, wherein the tertiary amine is a tertiary ethanolamine selectedfrom the group consisting of: N,N-Dimethylethanolamine (DMEA);N,N-Diethylethanolamine (DEEA), N,N-Diisopropylethanolamine (DIPEA);2-(dimethylamino)-2-methyl-1-propanol (2-DMAM-PrOH); and combinationsthereof.
 10. The system of claim 7, wherein said binding organiccompound is a liquid.
 11. The system of claim 7, wherein said bindingorganic compound further comprises a polar/aprotic non-aqueous solventof up to about 50 wt % selected from the group consisting of:dimethylsulfoxide, dimethylformamide, acetone, and combinations thereof.12. The system of claim 7, wherein the zwitterionic product is analkylcarbonate.
 13. A pressure swing reversible acid gas capture agenthaving the structure: wherein n is any carbon based chain and R is acarbon based chain or a carbon containing alcohol in the absence ofwater.


14. A Method for removing acid gasses from a gaseous volumecharacterized by the step of: contacting a gaseous volume containing anacid gas with at least one acid gas binding organic compound thatchemically binds with the acid gas to form a zwitterionic product at apressure greater than 100 psi; removing said zwitterionic product fromsaid gaseous stream or volume; and exposing said zwitterionic product toa lower pressure to release said acid gas and regenerate said acid gasbinding organic compound.